Background Art [0002] Silver (Ag) is one of the most electrically conductive materials in the world and is on par with the conductivity of gold (Au) and copper (Cu). Silver and gold, along with palladium (Pd) and platinum (Pt), are precious metals. Copper, nickel (Ni) and aluminum (Al) are considered to be base metals while tungsten (W) and molybdenum (Mo) are considered to be refractory metals due to their extremely high melting points. These precious, base and refractory metals would all benefit from an effective de-agglomeration and densification process.

The above materials are all employed in a variety of electronic applications to include: screen-printed conductive patterns on various substrate materials including alumina (A1a03) for hybrid microcircuits and integrated electronic packages such as low temperature cofired ceramic tape (LTCC); electrodes and terminations for chip components, such as ceramic chip capacitors and resistors; screen printed metallizations on silicon (Si) wafers (solar cell applications); and coatings on Si chips in one form or another (e. g. die attach to heat sinks for the semiconductor industry). Broad-based applications by market segment include circuitry for automotive, medical, military, semiconductor, consumer, solar and telecom markets. More specifically, examples of applications include conductive elements for cell phones, radar systems, stereo components, pacemakers, defibrillators, radar systems, smart munitions, GPS systems, and power surge systems.

[0003] The most versatile and popular material for all of the above applications is silver due to its excellent conductivity, low cost, availability and compatibility with other circuit elements. The primary form of Silver is a powder so that it can be incorporated into thick film screen-printable formulations. Fine, well-dispersed or de-agglomerated powder (micron-size range) is ideal so that fine circuit patterns and thin layers can be achieved. Most powder processes involve precipitation of some sort using metal salts and the resultant material is usually agglomerated and porous (both intra-and inter-particle porosity).

[0004] Dense powder leads to greater conductivity as porosity reduces the speed and density of the transfer of electrons. More dense powder also leads to better power handling capability in the various applications and reduced sensitivity to environmental conditions such as humidity, temperature and bias voltage. Silver, in particular, is sensitive to these conditions because silver, more than any other conductor, tends to migrate, i. e. , to form Ag dendrites under humidity and bias voltage conditions which lead to shorting phenomena. Thus, dense fine Ag powder is ideal.

[0005] Typical methods currently used for densification of metal powders include: ball milling; jet milling (which is usually done in a nitrogen jacket due to Ag oxidation issues and becomes very expensive under these conditions); attritor milling; and horizontal bead milling. These processes have shortcomings in the metal powder densification arena that can be related to inefficiency to include: low yields which are not acceptable when working with expensive precious metals; introduction of contaminants; and alteration of the starting morphology. An example of altered morphology is the production of flakes from spheres or irregularly-shaped particles due to repetitive impact of some processes.

In most applications, this flaking or flattening outcome is not desirable as a goal, although flaking or flattening is a goal in certain applications. In applications where flaking or flattening is not a goal, the goal is to de-agglomerate in order to increase the tap density of the material while maintaining the generally spherical shape of the particles (not alter the physical characteristics such as the morphology of the powder). In particular, the material before processing is agglomerated such that several primary particles are grouped together, creating lager units of material with gaps formed therein. It is desirable to de-agglomerate these larger units into primary particles without changing the morphology. In some applications where flaking or flattening is desirable, it is advantageous to de-agglomerate the material first, such that a less porous material is flattened, thereby resulting in a more dense flake or flattened particle, improving characteristics such as conductivity.

[0006] U. S. Patent Nos. 6,318, 649 and 6, 435, 435 relate to the utilization of high- pressure liquid jet technology for comminution and are incorporated in their entirety by reference herein.

BRIEF SUMMARY OF THE INVENTION [0007] The present invention is directed to a high G mill having a revolving disc spun by a high speed spindle. Material enters channels in the revolving disc and due to centrifugal force created by the high rotational speed thereof, the material is forced out of a nozzle located at a respective end of each channel. The material, which is exiting each nozzle at high speed, is subjected to a high shearing force as it exits each nozzle, and then impacts against a collision ring.

The shearing force and impact against the collision ring cause the material to comminute, de-agglomerate, and densify.

[0008] The mill of the present invention is particularly useful for densification of materials, in particular, metal powders. Using the mill of the present invention, metal powders can be densified to a greater degree than conventional methods used for densification. However, the mill of the present invention is also useful for milling, comminution, and homogenizing materials.

[0009] The present invention further relates to a system and method for comminution and processing of solid particles of one or a multitude of materials.

The method includes introducing the materials into a mill for comminuting the materials into particles. The mill outputs a slurry comprised of the particles and a fluid. The slurry is introduced into a spray dryer. The spray dryer atomizes the slurry, which falls through a heat zone, vaporizing the fluid. A collector catches the dry, falling particles.

[0010] In another embodiment, the mill outputs a slurry and introduces the slurry into a hydrocyclone before the slurry is introduced into the spray dryer. The hydrocyclone sorts the milled particles by size, returning oversized particles to the mill.

[0011] In another embodiment, the mill outputs a slury, introduces the slurry into a spray dryer and introduces the dried particles into a cyclone, where the particles are sorted by size. Any oversized particles are returned to the mill.

[0012] In another embodiment, one or more additives is introduced into the mill to be processed with the material. The additives can be either solid materials or fluids, such as binders or solvents. The output from the mill includes the additive (s), the particles of the material, and may include a fluid if the material was introduced into the mill as a slurry. If the output from the mill includes a fluid, the output may be introduced into an hydrocyclone and spray dryer. The hydrocyclone sorts particles according to size, and the spray dryer atomizes the output, which falls through a heat zone, vaporizing the fluid. A collector catches the dry, falling particles which have been coated with the additive.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES [0013] FIG. 1 is a cross-sectional view of a mill of the present invention.

[0014] FIG. 2A shows a plan view of alternative embodiments for the channels of the revolving disc.

[0015] FIG. 2B shows a cross-section of an open channel in the revolving disc.

[0016] FIG. 3 shows a high G mill of the present invention as either an open circuit or a closed circuit.

[0017] FIG. 4 shows a high G mill of the present invention disposed within a hermetically sealed enclosure and an isolation enclosure.

[0018] FIG. 5 shows a double funnel that may be used with the mill of the present invention.

[0019] FIG. 5A shows a high G mill of the present invention with a suction device.

[0020] FIG. 6 shows a cross-section of the mill of the present invention including alternative locations for cooling channels.

[0021] FIG. 7 shows an alternative embodiment of the mill of the present invention with a second spindle for rotating the collision ring in a direction opposite the revolving disc.

[0022] FIG. 8 is a table showing densification of commercially available silver powders using conventional milling techniques as compared to the milling techniques of the present invention.

[0023] FIG. 9 shows a first embodiment of a system of the present invention for comminution and processing solid particles of a material.

[0024] FIG. 10 shows an embodiment of a spray dryer equipped with a collector and condenser.

[0025] FIG. 11 shows another embodiment of FIG. 9, including a hydrocyclone and a recycling line.

DETAILED DESCRIPTION OF THE INVENTION [0026] FIG. 1 is a cross-sectional view of the high G mill 100 of the present invention. The mill comprises a revolving disc 101, a base 102, a cover 103, a collector 105, an outlet chute 111, a collision ring 106 and a funnel 107. The mill is connected to a high RPM spindle 110. Spindle 110 may be any commercially available high RPM spindle. A preferred spindle 110 utilized in conjunction with the present invention is available from Paul Müller GmbH & Co. and is marketed under the name GMN. An example of a spindle 110 utilized with the present invention is the GMN TSSV 120S-60000/SR spindle, with a maximum RPM of 60,000. However, conventional spindles from various companies, such as Fisher USA, could be utilized. Further, the maximum RPM may be as high as is available, for example, up to 200,000 RPM. The maximum RPM is limited only by the ability of the materials of the mill to withstand the high forces associated therewith.

[0027] More specifically, base 102 of the mill is coupled to spindle 110. A shaft 112 of spindle 110 is coupled to revolving disc 101 and provides the torque to spin revolving disc 101 at high RPMs. Revolving disc 101, mounted on shaft 112, is disposed between base 102 and cover 103, with space between base 102 and cover 103 to permit disc 101 to spin without interference. Funnel 107 is provided as part of or attached to cover 103. In one embodiment, threads on the outer surface of a neck portion of funnel 107 mesh with corresponding threads on an inner surface of a portion of cover 103. An outlet 114 of funnel 107 is disposed within an opening of disc 101.

[0028] Channels 116 are provided within revolving disc 101. Several channels 116 can be provided in revolving disc 101. For example, four radial channels 116 can be provided in the shape of a cross. Alternatively, eight radial channels 116 could be provided, spaced equally around the circumference of revolving disc 101 in a spoke pattern. Although the channels 116 have been described as radial channels, channels 116 need not be straight. Channels 116 may include gentle bends or sharp curves, as shown in FIG. 2A, for example. Other similar shapes may also be used for channels 116. It would be appreciated that as FIG. 1 is a cross-sectional view of the mill of the present invention, only two channels are shown, the section being taken where two of the channels span the diameter of revolving disc 101. A nozzle 108 is provided at the outer end of each channel 116.

[0029] Channels 116 have been described and shown as an opening in the middle section of disc 101, such that portions of disc 101 are above and below channels 116. Channels 116 can also be formed as open channels or grooves 117 in disc 101, as shown in FIG. 2B, such that there is no portion of disc 101 above the groove.

[0030] A collision ring 106 is provided outside the periphery of revolving disc 101. Collision ring 106 is sandwiched between cover 103 and base 102. As FIG.

1 is a cross-sectional view, it would be appreciated by those of ordinary skill in the art that collision ring 106 extends around the entire periphery of revolving disc 101. A gap 118 is located between the outer end of revolving disc 101 and collision ring 106. As can be seen in FIG. 1, collision ring 106 on the left hand side of the section is different from collision ring 106'shown on the right hand side. In practice, collision ring 106 or 106'would be identical around the periphery of revolving disc 101. However, two possible alternatives of the collision ring have been depicted in FIG. 1.

[0031] In particular, collision ring 106 shown in FIG. 1 is narrow. Therefore, gap 118 between collision ring 106 and revolving disc 101 is relatively large. In the embodiment shown on the left hand side of FIG. 1, a portion of gap 118 is filled with a gap ring 104. Gap ring includes a spacer 109 to create a space within gap ring 104. The space created is preferably around 20 microns, but may vary depending on the material being processed in the mill.

[0032] As also shown in FIG. 1, collision ring 106'is larger than collision ring 106. In such an embodiment, gap 118'is smaller than gap 118. Also in this embodiment, a gap ring 104 is not provided.

[0033] Different forces act on the material processed through the mill described herein. For example, a shearing force acts on the material as it exits nozzle 108.

The shear force acting on the material during exiting moment depends on the disc's diameter and RPM. Another force acting on the material is an impact force against collision ring 106. If gap 118, 188'is small, the impact force is larger than if gap 118,118'is large. Further, after exiting nozzle 108 and impacting against the collision ring, the material rebounds and hits against the outer periphery of revolving disc 101. Depending on the size of gap 118,188', the material continues to rebound between the outer periphery of disc 101 and collision ring 106,106'. The number of rebounds for a particular disc diameter and RPM depends on the gap size between the ring and disc. The more rebounds that occur, the more effective comminution is made. A bigger gap secures less rebounds and smaller gap more. The gap is the technological parameter which must be defined for particular material and selected process, as would be understood by one of ordinary skill in the art.

[0034] In operation, spindle 110 spins revolving disc 101 at high RPM. For a particular mill and spindle configured as described herein, one of ordinary skill in the art can determine the critical frequency of the mill, and this critical frequency should not be exceeded. In an example of the mill described herein utilizing the GMN spindle referenced above, the operational frequency was determined to be 43,200 RPM. The referenced spindle is rated for a maximum RPM of 60,000. However, with addition of disc 101, the maximum RPM drops due to the added weight of the disc. Further, the operational frequency should be safely below the critical frequency. One of ordinary skill in the art can determine the operational frequency of the spindle based on the factors noted above.

[0035] As revolving disc 101 is spun by spindle 110, material is inserted into the mill through funnel 107 directly into channels 116 of revolving disc 101. Due to the high speed of revolving disc 101, the material is forced through channels 116 at high speed, then out nozzles 108. Upon exiting the nozzle, the material collides against collision ring 106 or 106'. In the embodiment shown on the left hand side of FIG. 1, the material passes through the space in gap ring 104 created by spacer 109 before colliding with collision ring 106. As noted above, however, such a gap ring 104 is optional. After colliding with collision ring 106 or 106', the material exits the mill through collector 105 and output chute 111.

Depending on the material sought from the mill, the material collected at collector 105 can be routed back to funnel 107 for additional processing.

[0036] An example of a system with automatic re-processing of material from a collector 302 is shown in FIG. 3. FIG. 3 shows two alternative systems for a high G mill of the present invention. The left-hand side of FIG. 3 shows an open system in which material is fed into mill 100 manually through a metering device 304. Metering device 304 precisely controls the rate of material being fed into mill 100 to prevent clogging of the system. As shown in the left-hand side of FIG. 3, in an open system, the material passes through mill 100 one time and is collected in a collector 302 after processing. If more than one pass through the mill 100 is desired, the material from collector 302 can be manually fed back into mill 100 through metering device 304.

[0037] In a closed system, as shown on the right-hand side of FIG. 3, a metering pump 306 draws material from collector 302 and feeds it back into mill 100 through funnel 107. Piping 308 or another similar device for the material to be carried from collector 302 is also shown in FIG. 3. Metering pump 306 also controls the rate of material fed into mill 100 to prevent clogging. Material can be processed through mill 100 several times, controlled by the number of passes through mill 100, the size of the resulting material collecting in collector 302, or other means that would be understood by one of ordinary skill in the art.

[0038] The material entering funnel 107 can be a slurry (mixture of powder and a liquid) or a dry powder. For example, certain chemically modified silvers do not mix well with water, thereby making a water/silver slurry difficult to obtain.

However, with the High G mill of the present invention, the silver powder (or other hydrophobic or chemically modified metal powders) may be inserted into the mill in its dry powder state. This is beneficial over standard mills, such as the F-mill described in more detail below, which require a mixture of the metal powder and a liquid in order to operate. Accordingly, the metal powder, such as silver powder, may be inserted dry into the mill of the present invention at atmospheric conditions. The material may be inserted into the mill of the present invention at other than atmospheric conditions, such as in a vacuum.

Alternatively, the metal powder may by mixed with an inert gas such as argon, nitrogen, xenon, or others, in order to reduce oxidation. If the metal powder is mixed with a liquid, liquids that could be used in the mill include: water; oil; cryogenic liquids including carbon dioxide, nitrogen and helium; liquified gases including liquid carbon dioxide and liquid nitrogen; alcohols; silicone-based fluids including perfluoro carbon fluids; supercritical fluids including carbon dioxide or inert gas such as xenon or argon in a supercritical state; or organic solvents. One of ordinary skill in the art would appreciate that providing the material in its drypowder state at atmospheric conditions is the least complicated, but that combining the material with gases or fluids, or changing the conditions, may be desirable for other reasons.

] As noted above, it is beneficial for certain hydrophobic materials to be processed in a dry powder state. In such circumstances, as also noted above, it may be beneficial to process the material in other than atmospheric conditions, such as a vacuum, or to mix the powder with an inert gas such as argon, nitrogen, xenon or others, in order to reduce oxidation. FIG. 4 shows the mill 100 of the present invention in a hermetic box 402 for such vacuum or inert gas operations.

Further, a second, or isolation, box 404 is shown for temperature stabilization in cryogenic gas or liquid applications.

] FIG. 5 shows a variation of a standard funnel which is useful for introducing hydrophobic powders to be processed through mill 100 with a liquid.

FIG. 5 shows a double funnel 500 comprising an inner portion 502 and an outer portion 504. Double funnel 500 may also include a valve 506 within inner portion 502 for powder flow adjustment. As shown, the powder is inserted through inner portion 502 and the liquid is inserted through outer portion 504.

The powder and liquid mix at the opening of disc 101. As would be recognized by one of ordinary skill in the art, the powder and liquid may be switched such that the powder is processed through outer portion 504 and the liquid is processed through inner portion 502 of double funnel 500. In such a situation, valve 506 would need to be modified in order to control powder flow through outer portion 504.

[0041] FIG. 5A shows a suction device 508 that maybe used in conjunction with mill 100 of the present invention. Suction device 508 is coupled to outlet chute 111 of mill 100 and is particularly useful in applications where a dry powder is processed in mill 100. Suction device 508 transfers the processed powder directly to a sealed container using suction to contain the output of mill 100.

[0042] FIG. 6 shows a cross-sectional view of an alternative embodiment of mill 100 of the present invention. FIG. 6 is similar to FIG. 1, except that it shows cooling channels added to the mill. In particular, due to the impact of the material against collision ring 106, and other factors, mill 100 can get quite hot.

FIG. 6 shows two alternative locations for cooling channels to prevent mill 100 from overheating. In one embodiment, cooling channel 602 is provided within collision ring 106'. Although only shown on one side of FIG. 6, it is understood that cooling channel 602 would be provided around the entire collision ring 106, 106', or to only portions thereof, as appropriate. It is preferred that cooling channel 602 extend around the entire collision ring 106, 106'in order to provide a consistent temperature profile throughout. Alternatively, cooling channels 604 may be provided in cover 103. A pump (not shown) or other similar device is used to pump cooling fluid through cooling channels 602,604. It is understood that cooling channels 602,604 may be used jointly, or cooling channels may be located only in collision ring 106, 106'or only in cover 103. Further, although not shown, it is recognized that cooling channels may be provided in other portions of mill 100 to achieve the same result, such as in base 102.

[0043] FIG. 7 shows another embodiment of high G mill 100 of the present invention. In this embodiment, the parts are the same as in FIG. 1, except for the differences noted herein. In particular, in addition to spindle 110, a second spindle 702 is provided. Spindle 110, as described with respect to the embodiment shown in FIG. 1, spins revolving disc 101. Spindle 702 spins cover 103, which in this embodiment, is coupled to collision ring 106, so that collision ring 106 rotates in a direction opposite the direction of disc 101. For example, disc 101 may spin in a clockwise direction and collision ring 106 may spin in a counter-clockwise direction. This counter-rotation of disc 101 and collision ring 106 essentially doubles the velocity of the collision of material as it exits channels 116 and collides against collision ring 106. It is recognized that alternative methods of spinning collision ring 106 are possible other than coupling collision ring 1 06 to cover 103.

] The mill described above with respect to FIG. 1 is particularly useful for densification of materials, in particular, metal powders. The mill can be utilized to de-agglomerate and densify metal powders in a single process step with minimal contamination and virtually a 100% yield. This process beneficially does not substantially alter the morphology of the powder and has been shown to already surpass densification yields using conventional methods in the particular case of silver powder. The aforementioned high G milling process can be integrated into a powder production line prior to any drying steps conventionally required before post densification.

] The parameters of the mill, such as the rotational speed of the spindle, may be altered to vary the output of the mill. For example, as noted above, morphology of metal powders processed in the conditions noted above is not substantially altered. However, as noted above, in some applications, altering the morphology of the material, such as flattening or flaking, is desired. The operation parameters of the mill of the present invention may be altered, such as by substantially increasing the RPM, to alter the morphology of the material.

Particularly beneficial would be the process of de-agglomeration as discussed above combined with subsequent flaking or flattening in the same device.

The high G milling process described above has been tested using commercially available silver powder. FIG. 8 shows a table with results from the above-described process. In FIG. 8, the column 800 indicates the type of milling process used. For example, the"F-mill"is a conventional high pressure liquid jet comminution device, as described in U. S. Patent Nos. 6,318, 649 and 6,435, 435.

"MFR"is used to indicate that the material was processed by a commercial supplier of metal powders. The"High G"mill is the mill of the present invention.

Also in FIG. 8, column 805 indicates the various commercial powders tested.

Column 810 indicates the batch/test number of the product. For products where the raw product was processed by the supplier, "MFR"is inserted in the columns indicating the milling process used and"typical"is inserted in several columns indicating that the results are of typical batches using the supplier's process, representing a reasonable state of the art.

[0047] Column 815 indicates the dry weight in grams of the product used in the High G mill of the present invention. Column 820 the number of passes through the mill. Column 825 indicates the feed rate into the mill. Column 830 indicates the incoming tap density (TD) of the product, that is, the tap density of the product before undergoing the conventional processes or the process of the present invention. Similarly, column 835 indicates the apparent density of the incoming product. Columns 840,845, and 850 indicate the apparent density, the tap density, and the specific surface area after processing, respectively. Column 855 indicates the loss on ignition after processing. Columns 860,865, 870, and 875 are particle size designations based on the percentage of particles below a certain size in the processed product. For example, column 860 marked as "D100"indicates that 100 percent of the processed particles are smaller than the size indicated. Similarly, column 865 marked"D90"indicates that 90 percent of the processed particles are smaller that the size indicated. The size is given in microns. Column 870 marked"D50"is also referred to as the average particle size.

[0048] The results of FIG. 8 indicate that the process of the present invention produces more dense silver powder than conventional techniques. For a good comparative example, Batch #13 shown in the chart started with Product A having a tap density of 0.8 grams per cubic centimeter (g/cc) and an apparent density of 0. 52 g/cc. After processing by conventional techniques, the tap density was 2.22 g/cc and the apparent density was 1. 26 g/cc. Comparatively, using the process of the present invention at 20 passes, the resulting tap density was 3.52 g/cc and the resulting apparent density was 1.74 g/cc. Using the process of the present invention at 50 passes, the resulting tap density was 4.49 g/cc and the resulting tap density was 2.09 g/cc. Similar improvements can be seen in the chart of FIG. 8 for properties such as the average particle size (column 870), and other properties. Similar improved results can also be seen in FIG. 8 when comparing the results of the"F"mill and"typical"processes to the process of the present invention.

[0049] Although the test results indicated in FIG. 8 are specific to silver powder, the process can also be used on other precious metals and alloys (such as Ag/Pd), as well as base metals and refractory metals.

[0050] FIG. 9 shows an embodiment of a system 900 for comminution, blending and processing materials into particles. System 900 includes a high pressure pump 902, connected to a mill 904. Mill 904 has attached a feed pump 906 for introducing particles into a spray dryer 908. Connected to spray dryer 908 is a condenser 910 and a collector 912. A recycling circuit 914 connects condenser 910 to high pressure pump 902. However, it would be apparent to one skilled in the relevant art that various configurations of these elements could be used to implement system 900 of the present invention.

[0051] Mill 904 could be any mill capable of comminuting a material and can contain feeders to introduce solid material, additives, and/or binder into the mill, as would be apparent to one skilled in the relevant art. For example, a fluid energy mill as described in U. S. Patent No. 6,318, 649 and U. S. Patent No.

6,435, 435 can be used, or the high G mill 100 described herein can be used. In the case of the high G mill 100, a pump 902 is not necessary. Feed pump 904 or other device attached to outlet chute 111 takes the material exiting the mill for further processing.

[0052] System 900 is useful forcomminutingand/orblending orprocessingmore than a single material. For instance, an additive could be introduced to system 900 for blending with the particles in mill 904 for powder processing. The additive could be a solid material, added to the mill to be comminuted with the primary material, or the additive could be a fluid, such as a fluid binder for powder processing or a solvent.

[0053] In one embodiment, mill 904 is designed to achieve ultra-fine particles having a resultant size, also referred to as a product size, of less than 15 microns.

Preferably, the ultra-fine particles have a product size within a range of 1-5 microns. Still more preferably, the product size of the ultra-fine particles resulting from the use of mill 904 is below 1 micron.

[0054] It would be apparent to one skilled in the relevant art that mill 904 could be used to process a variety of other materials, both organic and inorganic, having various feed sizes. For example, mill 904 could be used to process any of the following: ceramic materials such as ceramics for electronic applications, such as, for example, barium and strontium titanates, lead zirconates, and other titanate and ziconates, and high temperature ceramic superconductors; ceramics for structural applications, such as, for example, alumina, zirconia, magnesia and other oxide systems; non-oxide ceramics, such as, for example, nitrides, borides, silicides and carbides used for toughness and abrasive applications; carbon and carbon by products (including coke and coke byproducts), minerals such as for example, anthracite, magnetite, alumina, mica, silica, and zircon, metals such as, for example, chromium, nickel, zinc, copper and brass, used for powder metal sintering, alloying and other applications, other oxides used for abrasive and cutting uses such as garnets, and rare earth oxides, and any other material that needs to be finely ground. Further, mill 904 could be used to process a variety of organic materials, including, for example, wood, food products and products for use as pharmaceuticals.

[0055] The size of material added to mill 904 for comminution may vary greatly depending on the material used, the type of mill, the size of channels 116 (if a high G mill 100 is used), the desired output, and other factors understood by one of ordinary skill in the art. Particles larger than 1/2"in diameter to less than a micron maybe introduced into the mill 904, depending on the features of the mill itself.

[0056] In one embodiment, the particles to be comminuted in mill 904 are dry as they are fed into the mill. In another embodiment, the particles could be fed into mill 904 as part of a slurry, e. g. , a mixture of material particles and a fluid. The fluid could be aqueous or non-aqueous, such as, for example, water or organic fluids such as alcohols and oils.

[0057] Introduction of a solid additive to mill 904 produces a mixture of comminuted material and additive. The material and the additive are simultaneously comminuted and aid in the breakdown of each other. The additive can be introduced at different stages of the mill to achieve different comminution ratios. As such, the resultant mixture can contain particles of additives that are larger, or less comminuted than the particles of material. For example, if the additive is introduced at the beginning stage of the mill, along with the material, the additive and the material may have roughly the same size reduction ratio, as both the material and the additive are subjected to the same milling process. Likewise, if the additive is introduced at a midstage, or toward the latter stages of the mill, the additive will be subject to less comminution, and therefore, may have a different size reduction ratio. This is, of course, also dependent on the initial sizes of the material, the material properties and the type of comminution stages used in the mill, as would be apparent to one skilled in the relevant art. Further, in the use of a high G mill 110 described above, the additive may be introduced at different times if the material is processed more than once through mill 100. For example, in the closed system described with respect to FIG. 3, the material may be processed several times and the additive may be added just prior to the final pass through the mill. In the alternative, the additive may be added prior to the first pass, for example.

[0058] Simultaneous comminution of the material and the additive in the mill provides intimate and intense mixing. As the material particles collide and contact the additive particles, the particles comminute into an evenly-blended mixture of material and additive particles.

[0059] The additive and the material each can be fed into the mill at a desired rate to achieve desired proportions and to create a properly proportioned mix. Such mixing eliminates the need for additional mixing or adding materials after the comminution process to obtain a desired proportion.

[0060] As stated, the additive could also be a fluid, such as a solvent or a binder.

When the additive is a fluid, it can be introduced as a secondary stream into the mill if a first stream of milling fluid is already in use. The additive can be introduced as a fluid stream at the front end of the fluid energy mill, such as at high pressure pump 902 or at funnel 107 of high G mill 100, or introduced further along in mill 904 in one of the succeeding chambers of a fluid energy mill. When such an additive is introduced with the material into the main chamber of a high pressure fluid energy mill, the fluid additive evenly coats the fractured material surfaces as they are formed.

[0061] In manufacturing processes requiring binders, it is important that nearly every individual grain of the particle is coated so that a resulting product, formed from the particles and binder, will exhibit consistent material properties. For ceramics, it is advantageous to compress coated particles into a dense compact, with the density approaching the theoretical density of a solid ceramic. The compact is sintered, resulting in a finished ceramic virtually devoid of holes and gaps. Thus, it is important that the coating on the particles be thin, as a thick coating, or excess binder, will leave gaps and holes in the resulting product when the binder is removed from the product during sintering.

[0062] In order to obtain a thin coating of binder, the binder must be spread evenly over the individual solid particles with a minimum possible thickness.

This is achieved by either lowering the viscosity of the binder or forcing the binder, under pressure, to penetrate and spread evenly over the particles.

However, the lower viscosity range of a binder is limited. Exceeding the lower limits of the range results in binder that segregates from the particles, dripping down the product, and resulting in degradation of its lubricating properties required during subsequent powder compaction. It is advantageous to use as little binder as possible, while obtaining a sufficient coat on each particle. This reduces the occurrence of voids and gaps when the binder is removed during sintering.

[0063] Two suitable binders for use in a mill comminuting alumina powder are methyl-methacrylate ("MMA") and Cyanoacrylate. These binders are not water soluble, and therefore, are not subject to being dissolved if water is the energy transfer fluid used in a fluid energy mill. Furthermore, these binders exhibit the desired properties of having a fast polymerization with a controllable reaction.

MMA polymerizes under standard atmospheric conditions. Therefore, it is necessary to add an inhibitor. For example, adding up to 0. 1% of an inhibitor such as Topanol or Hydroquinone to the MMA solution will prevent polymerization at atmospheric pressure.

[0064] It is generally preferable that the binder material not be dissolved by the fluid used for energy transfer in a fluid energy mill. However, dissolution of the binder material is permissible if the binder has a lower vapor pressure than the vapor pressure of the fluid used for energy transfer in the mill. This allows the fluid to vaporize in the spray dryer and leaves behind the binder material to form a thin coat over the dried solid particles. Examples of binders that could be used in the present invention include but are not limited to: CARBOWAX Polyethylene Glycols, manufactured by Union Carbide Chemicals and Plastics Co. , Inc. of Danbury, CT and Methocelcellulose ethers, manufactured by DOW Chemical of Midland, MI. Polyvinyl alcohols can also be used. Examples of polyvinyl alcohols that could be used in the present invention include: Elvanol 75-15 manufactured by E. I. du Pont de Nemours & Co., Inc. of Williamton, DE ; and Airvol 205, manufactured by Air Products and Chemicals, Inc. of Allentown, PA. The energy transfer fluid could be, for example, water or an appropriate alcohol such as isopropanol, which have higher vapor pressures than the binders, and readily evaporate in the spray dryer leaving behind the dry powder mixture.

It would be apparent to one skilled in the relevant art that a variety of fluids could be used as the energy transfer fluid in the mill.

[0065] In most cases, chemical reactions or surface property alterations are not permitted to occur in the mill or the spray dryer. For example, in the case of barium or strontium titanates, water can alter the surface properties of the particles by introducing hydroxyl ions which are undesirable in high performance electronic parts fabricated from these materials, such as, miniaturized multi-layer capacitors, and various transducers. To avoid chemical alteration, isopropanol alcohol is used as the energy transfer fluid according to the present invention which is totally inert with respect to the above solids. An alcohol soluble binder such as CARBOWAXP can be added to the mill during comminution. The process described above can be also be used for lead zirconates, which is also water sensitive. Lead zirconates is used extensively to fabricate active electronic devices such as solid state pressure sensors.

[0066] In some cases, chemical reactions and surface property alterations are desirable. In these cases, a solvent can be used as the additive, rather than a binder, to initiate reaction of the materials in the fluid energy mill. The solvent is preferably miscible with the fluid used for energy transfer. Furthermore, the solvent could be used as the energy transfer fluid if high concentrations of solvent are required for complete dissolution.

[0067] It is advantageous to initiate reactions in the mill because of the thorough blending achieved in the mill. Further, as the particles are comminuted, they have a greater surface area/volume ratio, increasing the rate of reaction.

[0068] One example of such a reaction produces coal tar pitch. Coal tar pitch is used to manufacture numerous carbon products, such as, graphite electrodes for the metal industry, specialty graphite and refractories for the nuclear industry, and graphite fibers forhighperformance composites. A solvent, N-methylpyrrolidone ("NMP"), forms coal tar pitch from anthracite. The reaction rate of NMP and anthracite coal particles increases dramatically by applying high pressure and mechanical impact in a fluid energy mill. The extracted carbon in the resulting coal tar pitch is used to produce many carbon products including those mentioned above. In this case, a spray dryer may be used to reclaim the unused solvent and fluid through evaporation, condensation and recycling.

[0069] As shown in FIG. 9, system 900 includes a spray dryer 908 directly attached to the output of mill 904 via an optional feed pump 906. Mill 904 outputs a slurry containing comminuted particles of a material and a fluid. If an additive was introduced into the mill, the output will include the comminuted material, the fluid and the additive. As would be apparent to one skilled in the relevant art, the material and the additive could be comprised of more than one material or additive.

[0070] As shown in FIG. 10, spray dryer 908 is attached to feed pump 906, and is comprised of atomizing components, such as a nozzle 1004 and a heating chamber 1006. Typically, a spray dryer mixes a spray and a drying medium, such as air, to efficiently separate the particles from the fluid as the particles fall through the air.

[0071] There are four general stages to spray drying: atomizing, mixing, drying, and separation. First, the feed or slurry is atomized into a spray. This is accomplished by introducing the slurry to feed pump 906, which forces the slurry through atomizing nozzle 1004. The energy required to overcome the pressure drop across the nozzle orifice is supplied by feed pump 906.

[0072] Second, the spray is mixed with a drying medium, such as air. Air can be added through a blower via nozzle 1004, via an additional nozzle, or can be merely present in chamber 1006. As would be apparent to one skilled in the relevant art, other drying mediums could be introduced in spray dryer 908. For instance, when the fluid, additive, or material is oxygen sensitive, inert gases such as nitrogen can be introduced as the drying medium. If a gas is added through a blower, the gas can be injected into chamber 1006 simultaneously with the atomized slurry. A conventional method of introducing gas and slurry simultaneously uses concentric nozzles, where one nozzle introduces gas and the other nozzle introduces slurry [0073] Third, the spray is dried. Drying occurs as the atomized spray is subjected to a heat zone in chamber 1006 or, alternatively, a hot gas, such as air or an inert gas as described above, is injected into chamber 1006. Flash drying quickly evaporates the fluid from the slurry, leaving only the dry particles. The small size of droplets allows quick drying, requiring a residence time in the heat zone ranging from 1-60 seconds, depending on the application. This short residence time permits drying without thermal degradation of the solid material.

[0074] Fourth, the product is separated from the gas. As the particles continue to fall, they exit chamber 1006, accumulating in particle collector 912, located at the bottom of chamber 1006. The now vaporized fluid is exhausted, or alternatively, collected in condenser 910. The spray dryer by-products are vaporized fluid and dry particles.

[0075] Using a spray dryer in connection with a mill provides several advantages over conventional drying techniques. For instance, spray drying produces an extremely homogeneous product from multi-component solids/slurries. A spray dryer can evaporate the fluid from the slurry, leaving the additive and material.

If the additive is a fluid, drying temperatures are held below the degradation temperature of the binder. As the fluid evaporates, a very thin coating of binder polymerizes on each particle. After being dried in the spray dryer, the particles are sufficiently coated for molding into compacts for sintering. Additional processing is not necessary.

[0076] Furthermore, the resulting collected particles are fine, dry and fluffy.

Conventional techniques, such as boiling the vapor off the particles, leave clumpy conglomerates of particles and result in less thorough blending of additives. The spray dryer also dries particles much faster than drying by conventional techniques. A spray dryer quickly dries a product because atomization exposes all sides of the particles to drying heat. The particles are subjected to a flash dry, and depending on the application, can be dried anywhere between 3 and 40 seconds. Thus, heat sensitive particles can be quickly dried without overheating the particles. As drying begins, the vaporized fluid forms around the particle.

This"protective envelope"keeps the solid particle at or below the boiling temperature of the fluid being evaporated. As long as the evaporation process is occurring, the temperature of the solids will not approach the dryer temperature, even though the dryer temperature is greater than the fluid evaporation temperature.

[0077] An additional advantage is that the spray dryer can operate as part of a continuous process providing dry particles as they are collected, rather than having to collect particles and then dry them. This also allows for fast turn- around times and product changes because there is no product hold up in the drying equipment.

[0078] The volume of an acceptable chamber 1006 can be determined by the equation, (residence time) * (volume flow rate) = volume of chamber, where volume flow rate is the throughput. Additionally, because of a larger surface area per unit mass, finer particles normally require longer residence time to dry than larger particles. Therefore, residence time may be longer for the finer materials.

Also, materials having hydroscopic properties will require a longer residence time in chamber 1006. Increased temperature may also be used to accelerate drying of such materials.

[0079] The spray dryer can be used for drying any slurry, whether the slurry is comprised of particles of a material, an additive, and a fluid or comprised of only particles of a material and a fluid. Further, the spray dryer can be a standard spray dryer, known in the art of spray drying. Spray dryer manufacturers and vendors include companies such as U. S. Dryer Ltd. ofMigdal Ha'emek, Israel, Niro, Inc. of Columbia, MD, APV of Rosemont IL, and Spray Drying Systems, Inc. of Randallstown, MD.

[0080] A conventional spray dryer can be outfitted with condenser 910. Because all drying takes place in an enclosed chamber 1006, capture and condensation of the vapors is easily performed. Condenser 910 collects the vaporized fluid from chamber 1006 and allows the spent fluid to be recovered. Thus, spray drying offers a simple way to contain the vapors from the evaporated fluid. Fluid recycling circuit 914, as shown in FIG. 9, can connect condenser 910 to optional high pressure pump 902 located at the beginning of the mill. This allows condensed fluid to be recycled by returning the used fluid from the spray dryer to the mill. This reduces waste and contains the fluid, which is especially important when the fluid is a regulated product, such as isopropanol. Isopropanol can be used as the fluid in the mill, introduced into the spray dryer where it is vaporized, recondensed in the condenser and returned to the mill for reuse. In this way, the fluid vapors are contained without risk of releasing harmful vapors into the atmosphere.

[0081] If the fluid is water, the water can be released from the spray dryer as vapor, can be condensed to be discarded, or can be recycled through the fluid recycling circuit. It would be apparent to one skilled in the relevant art that a variety of fluids could be used as the fluid in the mill.

[0082] In another embodiment, the slurry is introduced from the mill directly into the spray dryer. This embodiment does not use a feed pump connected to the nozzle for atomizing. Instead, fluid restrictors are used at the mill outlet to maintain the high pressures in mill 904. The slurry bypasses feed pump 906 and is injected directly from the outlet of mill 904 into spray dryer 908. In order to achieve proper separation of particles and fluid in spray dryer 908, the fluid pressures at the outlet of mill 904 must be sufficiently high to achieve complete atomization of the slurry. By eliminating the need for a feed pump to introduce the slurry to the spray dryer, the system operates more economically.

[0083] FIG. 11 shows another embodiment of system 900 for comminution, blending and processing materials into particles. This embodiment includes a hydrocyclone 110 located between mill 904 and feed pump 906. Hydrocyclone 1110 can be located either before or after feedpump 906, but is preferably located before it. A second feed pump (not shown) can be used to introduce slurry from mill 904 to hydrocyclone 1110, or, the slurry can be introduced into hydrocyclone 1110 directly from mill 904, as shown in FIG. 11.

[0084] Hydrocyclone 1110 aids in classifying solid particles exiting mill 904 by separating very fine particles from coarser particles. The coarser particles are fed through a recycling line 1112 back into optional high pressure pump 902, to be reintroduced into mill 904 for further comminution and processing.

Alternatively, recycling line 1112 can feed the particles directly back into mill 904, or can remove the particles completely from system 900. As the particles are still under pressure from hydrocyclone 1110, recycling line 1112 is a tube or enclosed circuit, which transfers the particles to mill 904 or high pressure pump 902.

[0085] The slurry from mill 904 enters the hydrocyclone at high velocity through an inlet opening and flows into a conical separation chamber. As the slurry swirls downward in the chamber, its velocity increases. Larger particles are forced against the walls, dropped to the bottom, and discharged through a restricted discharge nozzle into recycle line 1112. The spinning forms an inner vortex which lifts and carries the finer particles up from the bottom of hydrocyclone 1110, before they exit the discharge nozzle, and propel them through a forward outlet to feed pump 906 or, alternatively, directly to spray dryer 908.

[0086] In another embodiment, hydrocyclone 1110 is a dry type cyclone, located after spray dryer 908. In this embodiment, particles are dried in spray dryer 908 and gathered in collector 912. The dryparticles are introduced from collector 912 into cyclone 1110, where the particles are sorted according to size. Cyclone 1110 operates substantially similar to the hydrocyclone described above, using a gas as the fluid in place of a fluid. Again, oversized particles are reintroduced into fluid energy mill 904 or high pressure pump 902 via recycling line 1112. Because gases normally have less surface tension than fluids, dry separation normally results in finer and more accurate size distribution.

[0087] Hydrocyclone 1110 can be a commercially available hydrocyclone used for classification, clarification, counter-current washing, concentration, etc. , of particles. Examples of hydrocyclone and cyclone manufactures are Warman International, Inc. of Madison, WI (CAVEZ@ Hydrocyclone Technology), PolytechFiltration Systems, Inc., of Sudbury, MA (POLYCLONt Hydrocyclone Technology), and Dorr-Oliver, Inc. , of Milord, CT (DORRCLONE HYDROCLONES).

[0088] Because hydrocyclone 1110 recycles the larger or more coarse fraction of material back to mill 904 for further size reduction, hydrocyclone 1110 assists in achieving a narrow size distribution of finished particles. Furthermore, hydrocyclone 1110 offers more intimate mixing of the particles and additives.

Residence time in hydrocyclone 1110 is typically short, and is a function of the processing rate, and the equipment size (volume). Thus, residence time = equipment volume/processing rate (volume/time). Typically, the residence time in hydrocyclone 1110 is less than 60 seconds, and is preferably from 2-50 seconds. Thus, use of hydrocyclone 1110 does not restrict the processing rate achievable in mill 904 and subsequent spray dryer 908.

[0089] Depending on the size and capability of the hydrocyclone, residence time will vary for a given processing rate. Therefore, a properly sized hydrocyclone must be used to efficiently comminute, blend and process particles. An improperly sized hydrocyclone could impose limits on the residence times in other components of system 900.

[0090] While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.